Herpesviruses are the causative agents of a wide range of diseases suffered by humans and animals. For instance, herpes simplex viruses, types 1 and 2 (HSV-1 and HSV-2), are responsible for cold sores and genital lesions, respectively; varicella zoster virus (VZV) causes chicken pox and shingles; and the human cytomegalovirus (HCMV) is a leading cause of opportunistic infections in immunosuppressed individuals.
Herpes viruses are complex double-stranded DNA viruses that encode the enzymes that directly mediate Viral DNA replication. Seven DNA replication-associated polypeptides are required for human herpesvirus replication. Six of these seven polypeptides show a high degree of homology across all studied human herpesviruses. These six polypeptides, when expressed by the virus, include a heterodimeric DNA-dependent DNA polymerase, a monomeric single-stranded DNA binding protein, and a heterotrimeric helicase-primase complex that exhibits DNA-dependent ATPase activity, helicase activity and primase acuvity. The seventh DNA replication-associated polypeptide does not display sequence or functional conservation and is involved in the initiation of lytic viral replication.
Without the function of each of the seven herpesvirus-specific DNA replication proteins, herpesvirus chromosomal replication will not initiate or propagate. This has been demonstrated in two ways for DNA replication in HSV-1. First, temperature sensitive HSV-1 strains have been developed and the complementation groups within these strains have been mapped on a one-to-one correspondence to the seven HSV DNA replication genes. Additionally, transient replication assays that utilized recombinant DNA plasmids containing single DNA replication genes have found that the presence of each of the seven genes was required for the efficient replication of a tester plasmid containing an HSV-1 origin of DNA replication.
More recently, the DNA replication genes in other herpesviruses (i.e., Epstein-Barr virus, cytomegalovirus and varicella zoster virus) have been sequenced. These gene sequences are largely homologous to the HSV-1 DNA replication genes. Furthermore, transient replication assays containing either an Epstein-Barr virus or cytomegalovirus lytic origin of DNA replication confirmed their identity. In varicella zoster virus (the human herpesvirus most closely related to HSV-1) DNA replication genes were found to be highly homologous to HSV-1 (&gt;50% at the amino acid level) and present at identical relative locations on the two viral chromosomes. Although no follow-up analysis on varicella zoster virus DNA replication genes has been presented to date, it is highly unlikely that differences in the varicella zoster virus and HSV-1 DNA replication programs exist.
From the above, it is clear that human DNA replication proteins are unable to substitute for the HSV-1 encoded enzymes. Otherwise, temperature-sensitive viral polypeptides would have been complemented by human counterparts and the defective viruses would have continued to grow and replicate, even at elevated temperatures. Similarly, in transient replication assays, H human proteins were capable of complementing any of the seven herpesvirus-encoded polypeptides, an absolute dependence on the presence of each of these herpesvirus DNA replicafion-specific genes would not have been observed. Therefore, inhibiting the activity of these virally-encoded proteins represents an effective way of preventing herpesviral replication.
The herpes primase enzyme plays a critical role in the herpesvirus DNA replication program. The observation that the gene encoding the herpes primase is not only essential for replication, but is also highly conserved across the range of known herpesviruses, underscores the importance of this enzyme in mediating viral chromosomal replication.
In the helicase-primase complex, two of the three polypeptides (e.g., the expression products of the UL5 and UL52 genes of HSV-1) promote catalysis of duplex DNA unwinding (helicase) and RNA primer biosynthesis (primase). The third polypeptide, encoded by the UL8 gene, appears to modulate primase activity. The assembled helicase-primase enzyme complex functions both in the initiation and propagaton stages of herpesvirus DNA replication. It is responsible for the synthesis of RNA primers necessary for the initiation of all new DNA synthesis by the herpesvirus DNA polymerase. Additionally, for DNA replication to proceed, duplex viral chromosomal DNA must first be unwound to the single-stranded replicative intermediate because the herpesvirus DNA polymerase is inactive on fully duplex DNA. The helicase-primase is also responsible for this important DNA unwinding event.
Known anti-herpes therapies have not focused on inhibiting the primase activity of the herpes helicase-primase. The most widely used anti-herpes agents to date are purine and pyrimidine nucleoside analogs, such as acyclovir and ganciclovir. These nucleoside analogues inhibit replication of viral DNA by their incorporation into a growing DNA strand. The nucleoside analogue-based inhibitors of HSV-1 growth have found only limited success and are not generally useful in treating recurring infections in the majority of patients. In addition, the infection of humans by other herpesviruses, such as varicella zoster virus or cytomegalovirus, show little or no responsiveness to nucleoside-based therapies.
The lack of broad spectrum anti-herpesvirus activity by the nucleoside-based therapies is not surprising because these compounds act by indirect biological mechanisms. Nucleoside analogues must first be activated to the nucleoside monophosphate by a virally-encoded thymidine kinase enzyme. It should be pointed out that only HSV and varicella zoster virus encode thymidine kinase enzymes. This may, in part, explain the inability to adapt nucleoside-based therapies to the treatment of other human herpesviruses. After initial phosphorylation, the nucleoside analogue monophosphate must be further phosphorylated to the triphosphate by human-encoded enzymes prior to its action. Ultimately, the triphosphorylated nucleoside analogue is incorporated into a nascent DNA chain during viral genomic replication, thereby inhibiting the elongation of that DNA chain by the herpes DNA polymerase.
The final incorporation step of the nucleosidebased therapies has been characterized as "competitive" because the herpes DNA polymerase does not display a preference for the activated nucleoside drug versus normal deoxynucleoside triphosphates. However, because the action of the DNA polymerase is not considered rate-limiting for herpesvirus DNA replication, the utility of nucleoside-derived compounds in treating herpesvirus infections is necessarily limited. Accordingly, the need for effective, safe therapeutic agents for treating herpesvirus infections continues to exist.
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